This study was conducted to examine the relations of metacognitive knowledge about writing processes, transactional writing beliefs and writing performance, and to test the mediating effect of transactional writing beliefs. Thinking about Writing Questionnaire, Writing Beliefs Inventory, and four types of writing tasks (i.e., narrative, descriptive, expository, persuasive) were administered to 123 tenth-graders. Correlational analyses and structural equation modeling were conducted to test research hypotheses. The results were as follows. First, metacognitive knowledge was positively correlated with transactional writing beliefs, which were positively correlated with the quality of written composition. Second, the complete mediation model fit the data better than the partial mediation model. Third, in the complete mediation model, the path coefficient of 「metacognitive knowledge → writing beliefs」 and that of 「writing beliefs → writing performance」 were significant. Fourth, the mediating effect of transactional writing beliefs was significant. These results imply that transactional writing beliefs play the role of the generative mechanism that mediates the link between metacognitive knowledge and writing performance. The implications of the present findings for planning writing interventions and the suggestions for further research were offered.